This invention relates to a switching power supply characterized by minimizing a decrease in efficiency when load power is decreased, i.e. Minimizing switching loss under low load.
FIG. 4 shows a circuit diagram of a switching power supply of this type in accordance with a conventional technique.
In FIG. 4, switching element 310 switches and converts DC input voltages into AC voltages and provides stable output voltages via output transformer 360 to the secondary side of this transformer 360. Pulse width modulation (PWM circuit 320 controls the ON/OFF state of switching element 310 according to a feedback amount obtained by the detection of output voltages. Output voltage detection circuit 330 detects output state and feeds a signal back to the primary side of transformer 360 so that the output voltages are decreased when they are above a predetermined value and the output voltages are increased when they are below the predetermined value.
Photo coupler 340 is provided for the above-mentioned feedback purpose and transmits the signal obtained from output voltage detection circuit 330 to PWM circuit 320 on the primary side. Frequency variable oscillation circuit 350 changes oscillation frequencies according to feedback current so that the oscillation frequencies decrease at load power below a predetermined value and they increase at load power above the predetermined value. The primary side of transformer 360 is connected to power supply 370. Connected to the secondary side of transformer 360 are rectifier diode 380, rectifying capacitor 390, and load 400.
The conventional switching power supply configured as above controls switching frequencies by increasing or decreasing them according to the feedback amounts that vary with changes in output voltages whether they are above or below the predetermined value. This feedback amount is closely related to the period of time during which transistor 310 conducts (hereinafter referred to as xe2x80x9cON-timexe2x80x9d) that is determined by output voltage. Load power of the switching power supply is expressed by the equations shown below, where the ON-time of transistor 310 is T1, the input voltage is Vi, the output voltage is Vo, the period of time during which the back electromotive force of output transformer 360 is generated (hereinafter referred to as xe2x80x9cback electromotive generation timexe2x80x9d) is T2, the inductance of the transformer is L, the oscillation frequency is F, and the load power is P.
First, when the load power is above a critical current, i.e. when the load power is large, the transformer carries continuous current and the following equations hold:
Vixc3x97T1=Voxc3x97T2xe2x80x83xe2x80x83(1)
F=1/(T1+T2)xe2x80x83xe2x80x83(2)
When the load power is below this critical current and oscillation frequency F is constant, pulse widths T1 and T2 change with the crest values of the output and input voltages of the transformer windings. The crest value of the transformer output voltage is equal to that of the output voltage and the crest value of the transformer input voltage is equal to that of the input voltage. Direct current does not develop through the transformer windings.
Therefore, Vixc3x97T1xe2x88x92Voxc3x97T2=0, i.e. Vixc3x97T1=Voxc3x97T2, holds theoretically. Changes in power will not cause changes in pulse width; and when the frequency is constant, pulse widths T1 and T2 only relate to input voltage Vi and output voltage Vo and do not relate to load power P.
Consequently, as indicated by the waveforms generated under load above the critical current, as shown in FIGS. 5A and 5B, load power P has no relation with feedback amount (T1, T2). FIG. 5A shows a waveform generated when the load is large and FIG. 5B shows a waveform generated when the load is small. The difference between them is only the current waveform.
When the load power is below the critical current, i.e. when the load current is small and the transformer does not carry continuous current, the following equations hold:
P=(xc2xd)xc3x97Fxc3x97T22xc3x97Vo2/Lxe2x80x83xe2x80x83(3)
P=(xc2xd)xc3x97Fxc3x97T12xc3x97Vi2/Lxe2x80x83xe2x80x83(4)
Vixc3x97T1=T2xc3x97Voxe2x80x83xe2x80x83(5)
where equation (3) is based on the output voltage of the transformer seen from the output side and equation (4) is based on the input voltage of the transformer seen from the input side, and equation (5) shows a relation among input and output voltages and pulse widths.
Like these, equation (4) shows that output power P relates to the square of input voltage Vi, when oscillation frequency F and inductance L of the transformer are constant, and the feedback amount corresponding to ON-time T1 of the transistor is also constant.
Frequency control based on a feedback amount starts when the feedback amount is a predetermined value, i.e. when the feedback amount corresponding to ON-time T1 of transistor 310 is constant. Therefore, when input voltage Vi is low, the frequency control starts at low power; and when input voltage Vi is high, the frequency control starts at high power. Voltage waveforms generated when the load is below the critical current are shown in FIGS. 6A and 6B. FIG. 6A shows a case where the frequency control starts at high power when the input voltage is high and T1 is constant. FIG. 6B shows a case where the frequency control starts at low power when the input voltage is low.
The above-mentioned switching power supply has the following problem.
In a conventional switching power supply, load power P is obtained by the detection of a feedback amount. The feedback amount varies with output voltage Vo and input voltage Vi, when the load power is constant,. i.e. P in equation (3) is constant. Furthermore, when load power P and output voltage Vo are constant, i.e. output voltage Vo and pulse width T2 are constant, the feedback amount varies with input voltage Vi.
For this reason, when the frequency control based on a constant feedback amount is performed, since load power P is proportional to the square of input voltage Vi, the load power at which the frequency control starts is proportional to the square of input voltage Vi although such load power should be constant. Therefore, the frequency control is not performed at the load power determined ideal for starting it. Voltage waveforms generated when the load is below a critical current, T2 is constant, and input voltage Vi is different are shown in FIGS. 7A and 7B. As shown in FIG. 7A, when input voltage Vi is high, the frequency control starts at high load power; and as shown in FIG. 7B, when input voltage Vi is low, the frequency control starts at low load power. This has been the problem.
In order to address the above-mentioned problem, this invention aims to lower the switching frequency to a predetermined value at load power less than a predetermined value by detecting pulse waveforms of the output transformer, and to perform pulse width control when the frequency has been lowered to the predetermined value.
The load power under low load is closely related to the pulse width of the output transformer as shown by equation (4). Thus, detecting the pulse width from the pulse waveform of the voltage of the transformer winding provides the load power; and when the load is below a predetermined value, lowering the switching frequency will minimize a decrease in efficiency under low load.
As mentioned above, the present invention allows stable detection of predetermined load power regardless of variations of input voltages and improves power supply efficiency in a stable manner at load power below a predetermined value. At load power above the predetermined value, stable output can be supplied with the original power supply operation.